Transition detectors for detecting the presence of a transition in input or address signals have been widely used by designers to enhance circuit design performance. Simply stated, such transition detectors provide a pulse output having a predetermined duration whenever a transition is detected in a digital signal. One common implementation of such a circuit utilizes an exclusive-OR (XOR) logic gate and a time delay block, as shown in FIG. 1.
As a matter of background, the output of an exclusive-OR gate is a logic 1 when its inputs are at different logic states, and a logic 0 when its inputs are the same logic state. In the circuit of FIG. 1, one of the inputs of the exclusive-OR gate receives directly the signal being monitored. The other input receives the signal being monitored through the time delay block. The time delay block delays the signal by an amount .tau.. When a transition initially occurs in the signal being monitored, the inputs to the exclusive-OR gate have different logic states and the output of the exclusive-OR gate is a logic 1. After a time .tau., the inputs of the exclusive-OR gate attain the same logic value and the output of the exclusive-OR gate changes to a logic 0. This produces an output pulse having a pulsewidth .tau., which begins when the transition in the input signal first occurs.
In the past the time delay, .tau., has been generated using an integrated resistance and capacitance that provided an RC time constant.
One disadvantage of previous circuits of this type is the difficulty in controlling the time delay period due to variations in process and operating conditions. The duration of the time delay often varied unacceptably. Further, for most applications, it is desireable to have the length of the time delay related to propagation delays in other parts of the system in which the circuit is to be used. Typically, such a time delay would desirably vary in the same manner as the propagation delays varied over temperature, processing variations, and circuit aging. As such, it can be appreciated that prior transition detector circuits leave much to be desired.